Method of making radar reflector



p 1957 M. BRUCKER 2,805,974

METHOD OF MAKING RADAR REFLECTOR Original Filed June 20, 1952 @020 PLASTEI? MOLD APPLY s/mv METAL f'F/BPE GLASS CURE SAND BLAST COAT/N6 540x w/n/ saw/.0 UP APPLY A Z; A P '-;:g FIBRE GLASS BACK/N6 0F SHEETS 0F SHEETS HONEYCOMB FIBRE GLASS M/LTON BRUCKER,

INVENTOR.

United States at 2,805,974 METHOD F MAKENG RADAR REFTECTGR Milton Brucker, Los Angeles, Calif., assignor, by mesne assignments, to Zenith Plastics Company, Gardena, Califi, a corporation of Delaware 5 Claims. (Cl. 154-411)) This invention relates to a method for constructing a radar reflector. More particularly, it relates to a method of making a radar reflector of new and improved light weight and rigidity containing a minimum of metal while obtaining all the advantages normally attributed to allmetal reflectors. It likewise relates to an improved method of making the desired reflector, largely of plastic and fibrous materials.

' This application is a division of my earlier application Serial Number 294,632 filed on June 20, 1952 now U. S. Patent No. 2,747,180.

Heretofore commercially acceptable radar reflectors, for use in particular by the Armed Forces of the United States for aircraft, seagoing, and land use, depended upon use of an all-metal concavity, or the like, all-metal shield, either cast integrally as a reflective unit, or made reticulate by metal bars, rods, mesh, and the like. Such prior art devices are relatively heavy in weight, costly in their manufacture and maintenance, and include the use of metal predominantly if not exclusively. Inasmuch as the larger radar reflectors are most effective in use, the weight-size relationship becomes critical, particularly for aircraft use. It is also important, to a somewhat lesser extent however, on ocean-going vessels, and even on land.

In view of the above considerations and others, it is among the objects of this invention to provide a method of making, from readily available plastic and fibrous materials, a satisfactory and commercially acceptable radar reflector utilizing a minimum of metal but without thereby diminishing either the utility or durability of a radar reflector of comparable size made in accordance with prior art teachings.

It is another object of this invention to provide, in a radar reflector of the desired character described, a weather-resistant, non-metallic surface, a relatively fine layer of reflective metal, and a relatively still and substantial highly reticulate backing of utmost lightness consistent with strength and durability, and in a manner corresponding to the first mentioned non-metallic surface which is closely adherent to and provides a strong backing and protection to the reflective metal layer.

Yet another object of the invention is the provision of a. method of making a new and improved plastic construction for radar reflectors incorporating metal as the electronically eflective ingredient, and the plastic as a durable, weather-resistant and non-corrosive structural foundation.

Yet another object of the invention is to provide a method of making a plastic radar reflector construction capable of satisfying the rigid standards of the Armed Forces of the United States, as well as commercial requirements, and Whose utility is susbtantially unaffected by increases in size, said increase being accompanied by disproportionate increases in weight, foundation, and reinforcement.

It is among the objects of this invention to provide a new and improved method of making a radar reflector as herein described and claimed.

2,805,974 Patented Sept. 10, 1957 Yet another object of the invention is the provision of a new and improved method for insuring suflicient I'lgld-' ity of the metal reflector portion of the device of the 1n-- stant invention. I

It is moreover an object to provide an improved method for insuring permanent adherence of the several metal and non-metal parts of the desired structure to one another.

It is a further object of the invention to provide new and improved means and molding method for obtaining a radar reflector of the instant invention.

In addition, it is among the objects of this specification and invention to set forth a suggested, preferred means and method for obtaining a new and improved radar reflector construction in accordance with the inventors conception; to improve prior art devices and methods heretofore intended to accomplish generally similar purposes.

These and other objects and purposes will be more fully understood by reference to the accompanying specification considered in the light of the drawings and the appended claims.

In the drawings:

Figure l is a perspective view, partially in section, of a mold and radar reflector constructed thereon in accordance with the teaching of this invention.

Figure 2 is a flow diagram of the method of this invention.

Figure 3 is a fragmentary sectional view of one edge of a finished reflector made by the method of this invention.

Referring more particularly to the drawings, there is illustrated by way of example but not of limitation a mold is of plastic or the like, having a base 11 by which the same is supported in any suitable fashion, as upon a floor, and provided with any reinforcing rim, as of wood or steel, 12. The upper surface 13 of the mold is convex to conform to the desired reflective shape of the intended reflector surface. The mold is further preferably provided with an annular flange portion 14 for a purpose to be described.

In the construction of the instant reflector, a layer of fiberglass cloth 15, thoroughly impregnated with a resin, is closely laid over the upper surface of the convex portion 13 of the mold, and also to the upper surface 16 of the flange 14 thereof. The resin is preferably also applied, prior to the application of the impregnated fiberglass sheet 15, to the mold surfaces 13 and 16, as by means of a brush, a spatula, by spraying, or by hand.

Said resin is most desirably an unsaturated polyester selected from any of those commercially available products more specifically, styrene copolymers of unsaturated alkyd resins, e. g., alkyd made from diethylene glycol with maleic anhydride and phthalate, soluble in styrene, or from diallyl phthalate.

The purpose of the first layer of cloth 15 is to provide a weather-proof coating for the reflective side of the reflector, to protect the metal coating 17 from oil and corrosion, and to provide a body which is conveniently and easily removed from the mold upon completion of the product. For such purpose a fiberglass woven mat of approximately ten thousandt'ns of an inch thickness is used.

The initial layer of fiberglass 15, together with the resin impregnated therein and thereunder, is next cured as under an ultraviolet lamp, which assists the activity of the lightactivated catalysts, if any, which may be incorporated in the resin, within the skill of those familiar with the art, or such curing may be accomplished in any conventional fashion, as in an oven for approximately an hour, at approximately 200 degrees Fahrenheit, or other satisfactory temperature. I 1

After the curing of the fiberglass impregnated resin, the exposed upper surface of the fiberglass mat is subjected to a sandblasting. Such sandblasting provides a satisfactory surface for the; next operation, which is the application, by means ofa, metal spray, of alayer of metal of substantially uniform thickness over the entire convex surface of the mold and cloth layer 15. Alayer of aluminum has been found satisfactory forthe instant purposes if deposited, for example, in a layer pf approxi mately ten thousandths of an inch. Other metals may also be employed, but if so, they should be selected from those which. do not require such a high temperature for spraying that the resultant heat of the sprayed metal willdeleteriously or otherwise afiectthe resin.

To the upper surface of the metal layer 17 there is applied, with or without sandblasting, and preferably also but not necessarily, a coating as by brush, spatula, by spray or by hand, a layer of said resin, over which is similarly laid a fiberglass mat 18, preferably of woven fiberglass. Said layer 18 has been found satisfactory if comprised of three layers of such fiberglass fabric of about thirty thousandths of an inch combined thickness. Also satisfactory is a single fiberglass mat of woven fiberglass fibers of approximately the same total thickness.

Over the last named layer 18 is placed a preferably honeycomb core 19. If it be assumed in the instant example that the diameter of the reflecter is to be approximately eight to nine feet, a honeycomb core of approximately one and three-quarter inches in thickness has been found suitable. The honeycomb itself comprises woven fiberglass impregnated with a polyester resin. The individual cells of the core, by way of example, may be hexagonal and extend continuously from the inside to the outside walls of the core. The walls of the core defining the honeycomb-like chambers are approximately three thousandths of an inch in thickness and are impregnated with said resin to give them a total thickness, including said resin, of about five thousandths of an inch.

After the honeycomb has been positioned over the layer 18, preferably though not necessarily as a continuous unit, a further backing 20 of woven fiberglass fabric impregnated with said resin is laid over the .core 19. Such a backing has been found satisfactory. in the form of approximately four layers or plies of woven fiberglass, each of which ply has an approximate thickness of ten thousandths of an inch, the whole backing 20 thereby having. an over-all thickness of approximately forty thousandths of an inch.

After the positioning of the layers 18, 19, and 20, over the sprayed metal layer 17, the upper surface of the mold is covered with a flexible bag 21 of any material such as polyvinyl acetate, or the like, non-compatible material which 'does not, like rubber, inhibit curing of the resin,

and which does not, after such curing, adhere to the resin.

Said bag 21 is laid also over the upper surface of the corresponding layers 16 through 29 disposed upon the flange 14, and if desired, also over'the outermost edge 22 of the flange 12, but prior to the positioning of the bag 21, a porous or otherwise open tube 23 is coiled about the mold. Such tube 23 may take the form merely of a coil spring or other coil of material comprising a helix. Moreover, at the outer edges of the flange, as at 24 and 25, a layer of sealing compound, such as zinc chromate, is employed, having the desired quality that it does not flow under the contemplated heat to which the mold is to be subjected.

The bag 21 is pressed into position over the zinc chromate seal, which seal, however, is not deposited inwardly, as at 26, between the tube 23 and the convex portion 13 of the mold.

A vacuum is next interconnected with the tube 23. The vacuum heretofore employed has been that required to subjectth'e dome 27, or convexity, to a pressure of approximately nine pounds per square inch; While the vacuum is applied, any air under the dome portion 27 is rubbed out as by brushing, rubbing, rolling or the like,

with pressure against the top of the dome, beginning in an oven, under ultra-violet light, or otherwise.

Thereupon the cured part is removed from the mold by physical stripping after the bag 21 has been removed. The edges designated collectively at 28 overlying the flange 14 are trimmed off, and the raw edges 29 thus exposed are capped as by two layers 3.0.31 of woven fiberglass mat, each having a thickness of approximately ten thousandths of an inch, impregnated with said resin, as under a heat lamp, to approximately 200 degrees Fahrenheit, for approximately an hour.

A reflector made as above described, by Way of ex:

ample, may have a diameter ,of approximately nine. feet.

The over-all thickness, which may be substantially the.

same from edge to edge, isaPPIOXimateIy 1.84 inches, not counting the added thickness of the cap 30-131.

The versatility of the instant. construction is such that a.

reflector may be made in any diameter, as for example, from two inches to fifty or even feet in diameter. The curve of the reflector surface is preferably parabolic. The weight of a corresponding steel frame of effective nine-foot diameter is approximately 350 pounds; whereas, the corresponding weight of an effective nine-foot reflector made in accordance with this invention is only. ninety pounds. Even the cost of the instant reflector is substantially less than that of its steel counterpart.

A reflector made in accordance with the teaching of this invention has, under test, withstood a mile-per-hour gale directed axially at same, or while rotating the same, with less than one-eighth of an inch deflection while mounted on a standard support. Y

It has excellent Weather-resistant qualities and may 'be used either exposed or under the protection of a radome or like shelter, in aircraft, aboard ship, or as part of a stationary or mobile land unit.

Although I have herein shown and described my in vention in what I have conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of my invention, which is not to'be limited to the details dis: closed herein but is to be accorded the full scope of the claims so. as to embrace any and all equivalent structures and methods. e

' I claim:

1. The method of making a radar reflector comprising forming and retaining a layer of resinous material in a generally dish-shaped condition, curing said resin, sandblasting the outer' curved surface of said resin, spraying a metal uniformly over the sandblasted surface, permitting said metal to' harden on said surface, impregnating the" surface of said sprayed layer of metal with such thermosetting resin, impregnating a fiberglass cloth with said resin and applying the impregnated cloth to said impregnated sprayed metal surface, applying a cellular substantially rigid member on the outer surface of the impreg-.

nated fiberglass cloth layer, impregnating a second fiberglass cloth with said resin and applying said cloth over said cellular member, removing entrapped air and pressing.

all of said layers radially together and curing said resin in and between said layers, trimming the marginal edge:

of the resultant cured mass, impregnating a fiberglass material with a thermo-setting resin and applying said ma terial over said trimmed edge, and curing said last men-- tioned resin in place. 7 a

2. The method of making a radar reflector comprising forming and retaining a layer of woven fiberglass cloth in a generally dish-shaped condition, impregnating said cloth t a t me-s t e in e l psoid tesi is a i the outer curved surface of said resin-impregnatedcloth,

spraying a metal uniformly over the sandblasted surface, permitting said metal to harden on said surface, impregnating the surface of said sprayed layer of metal with such thermo-setting resin, impregnating a second fiberglass cloth with said resin and applying said impregnated second cloth to said impregnated sprayed metal surface, applying a cellular substantially rigid member on the outer surface of said second impregnated fiberglass cloth layer, impregnating a third fiberglass cloth with said resin and applying said cloth over said cellular member, removing entrapped air and pressing all of said layers radially together andcuring said resin in and between said layers, trimming the marginal edge of the resultant cured mass, impregnating a fiberglass material with a thcrmo-setting resin and applying said material over said trimmed edge, and curing said last mentioned resin in place.

3. The method of claim 2, wherein said thermo-setting resin is an unsaturated polyester.

4. The method of claim 2, including the additional steps of covering the foregoing layers with a flexible impervious layer of material non-compatible with said resin, and removing said air under vacuum around the circumferential periphery of said dish-shaped structure while pressing out any such entrapped air and residual resin in the direction of said circumferential periphery and said vacuum.

5. The method of making a radar reflector of the character described comprising forming a dome-shaped mold, positioning said mold with said dome shape uppermost upon a solid foundation, forming an annular flange around said dome at the base thereof, impregnating with a thermosetting resin and applying over said dome a layer of fiberglass cloth, curing said thermo-setting resin in and upon said cloth, sandblasting the upper cured surface of said resin, spraying a substantially uniform layer of aluminumlike metal over said sandblasted surface, impregnating with a thermo-setting resin and laying thereover a second fiberglass cloth, laying over said second fiberglass cloth a rigid material to a substantial thickness, impregnating with a thermo-setting resin and laying thereover a third fiberglass cloth, laying thereover an impervious flexible sheet, providing a vacuum under said last sheet substantially entirely around said dome at said flange, and wiping said dome downwardly toward said flange and vacuum while curing the resin, removing the cured reflector from the mold and trimming and finishing the edges thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,439,137 Keller Apr. 6, 1948 2,478,165 Collins Aug. 2, 1949 2,638,428 Gordon et al. May 12, 1953 2,706,832 Frost et al Apr. 26, 1955 FOREIGN PATENTS 365,755 Great Britain Jan. 28, 1932 

1. THE METHOD OF MAKING A RADAR REFLECTOR COMPRISING FORMING AND RETAINING A LAYER OF RESINOUS MATERIAL IN A GENERALLY DISH-SHAPED CONDIDTION, CURING SAID RESIN, SANDBLASTING THE OUTER CURVED SURFACE OF SAID RESIN, SPRAYING A METAL UNIFORMLY OVER THE SANDBLASTED SURFACE, PERMITTING SAID METAL TO HARDEN ON SAID SURFACE, IMPREGNATING THE SURFACE OF SAID SPRAYED LAYER OF METAL WITH SUCH THERMOSETTING RESIN, IMPREGNATING A FIBERGLASS CLOTH WITH SAID RESIN AND APPLYING THE IMPREGNATED CLOTH TO SAID IMPREGNATED SPRAYED METAL SURFACE, APPLYING A CELLULAR SUBSTANTIALLY RIGID MEMBER ON THE OUTER SURFACE OF THE IMPREGNATED FIBERGLASS CLOTH LAYER, IMPREGNATING A SECOND FIBERGLASS CLOTH WITH SAID RESIN AND APPLYING SAID CLOTH OVER SAID CELLULAR MEMBER, REMOVING ENTRAPPED AIR AND PRESSING ALL OF SAID LAYERS RADIALLY TOGETHER AND CURING SAID RESIN IN AND BETWEEN SAID LAYERS, TRIMMING THE MARGINAL EDGE OF THE RESULTANT CURED MASS, IMPREGNATING A FIBERGLASS MATERIAL WITH A THERMO-SETTING RESIN AND APPLYING SAID MATERIAL OVER SAID TRIMMED EDGE, AND CURING SAID LAST MENTIONED RESIN IN PLACE. 